1. Field of the Invention
The present application relates to a method to control an optical transmitter, in particular, the present application relates to an algorithm to set a bias current in a semiconductor laser diode (hereafter denoted as LD) depending on a temperature thereof.
2. Related Background Art
The optical communication system installs a transmitter optical sub-assembly (TOSA) to convert an electrical signal to be transmitted into an optical signal. A TOSA generally includes a light-emitting device, typically an LD and a light-receiving device, typically a photodiode (hereafter denoted as PD), to monitor output power of the LD. An LD has been well known as a semiconductor device having large temperature dependence in various control parameters thereof. For instance, an LD shows large temperature depended in the I-L characteristic, namely, the output power against the input bias current. Accordingly, the bias current and the modulation current supplied to an LD are necessary to be precisely adjusted depending on a temperature to keep an average power and an extinction ratio of the optical output in a wide temperature range.
Various techniques have been reported in prior arts. For instance, the bias current supplied to the LD, and the bias voltage and the modulation amplitude supplied to the electro-absorption (hereafter denoted as EA) modulator, where they are installed within a TOSA, is precisely adjusted depending on an inner temperature of the TOSA which is monitored by a temperature sensor such as thermistor installed within the TOSA. Another technique controls a temperature of a thermo-electric cooler (hereafter denoted as TEC) which is installed within the TOSA and controls a temperature of the devices mounted thereon.
However, a TOSA installing a temperature sensor is necessary to provide an additional lead terminal to extract a signal corresponding to the temperature generated by the temperature sensor from the TOSA. Such a TOSA providing additional pins is hard to make the TOSA in compact.